In the art of manufacturing pneumatic tires, rubber flow in the mold or minor differences in the dimensions of the belts, beads, liners, treads, plies of rubberized cords, etc., sometimes cause non-uniformities in the final tire. When non-uniformities are of sufficient magnitude, they will cause a tire to be imbalanced. Regardless of its cause, when the imbalance exceeds an acceptable maximum amount, the ride of the vehicle to which such an imbalanced tire is mounted will be adversely affected.
Essentially, two separate physical phenomena contribute to the imbalance of a tire, static imbalance and couple imbalance. Static imbalance is the result of net centrifugal forces created by non-uniformities in the distribution of tire mass about the rotational axis of the tire. Non-uniformity of tire mass distribution is caused by manufacturing variations which create mass distribution differences about the radius of the tire tread. As an element of tire mass rotates about an axis, centrifugal force is experienced by the element, which tends to push it away from the center of rotation, the magnitude of this centrifugal force being: EQU F=m.times..omega..sup.2 .times.r
wherein m=mass of the element, .omega.=rotational velocity, and r=radius of the circle of rotation. If the mass of the tire is distributed equally about the center of rotation, the centrifugal force on each of the elements of tire mass would be negated by an equal and opposite force acting upon an element of tire mass located on the opposite side of the center of rotation, and thus no net centrifugal force would act upon the tire during rotation. However, when the distribution of tire mass is nonuniform, so that there are elements of greater mass or elements located at greater radial distance from the center of rotation, the centrifugal force on these elements is not canceled by the opposing force acting on the element of tire mass located on the opposite side of the center of rotation. In such cases, the tire experiences a net centrifugal force acting through the element of either greater tire mass or located at a greater distance from the center of rotation. These net centrifugal forces cause a static imbalance about the center of rotation of the tire.
Couple imbalance is caused by the above described mass distribution non-uniformities, or mass imbalances, about the radius of the tire which create net moments about an axis in a plane which is through the centerline of the tread radius and perpendicular to the axis of rotation of the tire. The magnitude of such a moment equals the net force acting on the mass non-uniformity, or the imbalance force, multiplied by the distance of the mass non-uniformity from the centerline of the tread (and thus the axis located in the plane through the tread centerline). This moment can be expressed as: EQU M=F.times.d=(m.times..omega..sup.2 .times.r).times.d
wherein variables m, .omega., and r are the properties described above and d=distance between the mass non-uniformity and the centerline of the tread. The effect of such moments is that the tire tends to wobble along its axis of rotation. Couple imbalance is usually caused by mass distribution differences about the circumference of the tire, between the upper and lower plane, the planes being parallel and equally spaced from the plane through the centerline of the tread.
The combined effect of the static imbalance and the couple imbalance is referred to as the dynamic imbalance of a tire, which is the total imbalance experienced by a rotating tire. As static imbalance and couple imbalance are two distinct and mutually independent physical phenomena, the dynamic behavior of a rotating tire can be analyzed by overlaying the effect of static imbalance on the effect of couple imbalance. Virtually all tires have some differences in the distribution of the tire mass which causes dynamic imbalance to be present, but the imbalance will be negligible, or at least acceptable, in a uniform tire.
Among several force variations which cause problems with tire performance is tangential force variation, or fore-aft force variation which is experienced at the surface of contact between tire and road surface in a direction both tangential to the tire tread and perpendicular to the tire axis of rotation. Tangential force variations are very speed dependent and are experienced as "push-pull" effect on a tire, which can be analogized to the sensation of a wheel barrow traveling over a bump in the road, i.e. increased force as the wheel barrow is pushed up the bump and decreased force as the wheel barrow travels down the bump. Investigations have shown that there are multiple mechanisms active in causing tangential force variation. However, to date, the tangential force variation is essentially unmeasurable on a typical production low speed tire uniformity machine, as discussed in more detail below, which operates at a speed, such as 60 revolutions per minute (RPM). Instead, tangential force variation can only be measured at highway speed or above, using a high speed laboratory tire uniformity machine, such as a Model HSU-1064, available from the Akron Standard Co. of Akron Ohio. Because of the low productivity and expense of the laboratory tire uniformity machine, the tangential force variation parameter can only be measured by sample methods.
In the usual tire manufacturing process, tires are placed first in a production tire uniformity machine to correct force variation and then placed in a tire balance machine to measure time imbalance. Sufficiently large non-uniformities in a tire will cause, besides imbalance and tangential force variations as outlined above, other force variations on a surface, such as a road, against which the tires roll. These force variations produce vibrational and acoustical disturbances in the vehicle upon which the tires are mounted, and when such variations exceed an acceptable maximum level, the ride of a vehicle utilizing such tires will be adversely affected.
Consequently, there have been a number of methods developed to correct excessive force variation by removal of rubber from the shoulders and/or the central region of the tire tread by means such as grinding. Most of these correction methods include the steps of indexing the tire tread into a series of circumferential increments via computer control and obtaining a series of force measurements representative of the force exerted by the tire as these increments of tire tread contact a surface. This data is then interpreted and rubber is removed from the tire tread in a pattern generated by this interpretation.
Force variation correction methods are commonly performed with a production tire uniformity machine (TUM), which includes an assembly for rotating a test tire against the surface of a freely rotating loading wheel. In such an arrangement, the loading wheel is moved in a manner dependent on the forces exerted by the rotating tire and those forces are measured by appropriately placed measuring devices. When a tire being tested yields less than acceptable results, shoulder and center rib grinders are used to remove a small amount of the tire tread at precisely the location of non-uniformities detected by the measuring devices. As the tire is rotated, it is measured and ground simultaneously. In a sophisticated, low speed production tire uniformity machine, such as a Model No. D70LTX available from the Akron Standard Co. of Akron Ohio, the force measurements are interoreted by a computer and rubber is removed from the tire tread using grinders controlled by the computer. Examples of tire uniformity machines utIlizing these methods are disclosed in U.S. Pat. Nos. 3,739,533, 3,946,527, 4,914,869, and 5,263,284. Another tire uniformity machine utilizing similar methods is disclosed in U.S. Pat. No. 4,914,869, published on Apr. 10, 1990 and entitled METHOD FOR CORRECTING AND BUFFING TIRES. DE-A-4,436,200 published on Apr. 18, 1996 discloses a machine for grinding a tire.
Once a tire undergoes correction for force variations in a TUM, it is common manufacturing practice to remove the tire from the TUM and place the tire in a balance machine to measure the amount of imbalance of the tire. Typically, the tires are mounted in the balance machine in a manner similar to that of the tire uniformity machine and inflated to a preset pressure. Then, the static and couple imbalances are measured by one of a variety of well-known methods. When a tire is found to be imbalanced to an unacceptable level, the tire is ordinarily scrapped. In present, state-of-the-art tire manufacturing processes, the great majority of the tires measured have acceptable amounts of imbalance. Still, with the huge volume of tires produced annually, there are still a significant number of tires which are scrapped in the current manufacturing processes resulting significant waste of material and resources.
Because of the increased awareness of environmental and financial concerns, it has become increasingly important to find a means to correct the imbalance of tires found to be unacceptably imbalanced. Moreover, with improved drive suspensions and lighter automobiles, the requirement to correct for tangential force variation is expected to become a reality in the foreseeable future.